Which of the following reagents convert $\mathrm{CH}{}_3\mathrm{COOH}$ into $\mathrm{CH}{}_3\mathrm{COCl}$?

Thionyl chloride, phosphorus pentachloride and phosphorus trichloride all convert the OH of the acid into a good leaving group which is displaced by $\mathrm{Cl}{}^{-}$. Concentrated HCl cannot, as $\mathrm{Cl}{}^{-}$ cannot displace $\mathrm{OH}{}^{-}$ (and acid chlorides hydrolyse in water).